A disposable body warmer is a body warmer that utilizes heat generation caused by the oxidation action of iron powders. In the disposable body warmer, usually, an inner bag including an air-permeable bag of a non-woven fabric, paper, or the like which accommodates an exothermic composition containing iron powders as an exothermic element, a salt as an oxidation catalyst, activated carbon for intake of oxygen, water for oxidizing iron, a water-retaining agent for retaining the water, and the like, is accommodated and airtightly packaged in an outer bag made of an air-impermeable film cutting off contact with air in an unused state.
An outer bag for disposable body warmer packaging is required to be excellent in gas barrier property for inhibiting permeation of air, in particular, oxygen gas, water vapor, and the like. If the outer bag for disposable body warmer packaging is poor in its gas barrier property against oxygen gas and water vapor, any gas and water vapor in the outer bag for disposable body warmer packaging will escape outside during storage for a long period, producing a depressurized (vacuum) state and consequently causing the outer bag to be depressed, which is not preferable in terms of appearance. In addition, when such an outer bag in a depressurized (vacuum) state is stored for a long period and thereafter used as a disposable body warmer, the duration of heat generation thereof is often short. As an air-impermeable film forming the outer bag for disposable body warmer packaging, a multilayer film is generally used in which a gas barrier layer is provided on a sealant layer and a heat-resistant resin layer is provided as an outermost layer. Two of such air-impermeable multilayer films are stacked each other and the peripheries of the sealant layers located inward are heat-sealed to each other into the form of a bag. The outer bag for disposable body warmer packaging is thus manufactured.
Meanwhile, it is known that a trace amount of hydrogen gas is generated when a disposable body warmer is stored in an unused state for a long period. The outer bag for disposable body warmer packaging becomes swollen due to the hydrogen gas, and this is not preferable in terms of appearance. In addition, the outer bag is broken when the swelling reaches its limit, and an exothermic composition accommodated in an inner bag is oxidized to generate heat, thereby causing the function as a disposable body warmer to be impaired. For this reason, hydrogen gas permeability is required to such an extent that the outer bag for disposable body warmer packaging is not swollen during the long storage period. However, as described above, it is also required that the outer bag for disposable body warmer packaging excel in the gas barrier property for inhibiting permeation of oxygen gas, water vapor, and the like. As a result, when the bas barrier property of the air-impermeable film forming the outer bag for disposable body warmer packaging is made higher, the hydrogen gas permeability is decreased, causing swelling during the long storage period. As an outer bag for disposable body warmer packaging that takes the hydrogen gas permeability into consideration, an outer bag for disposable body warmer packaging has been proposed which includes a plastic film and a vapor-deposited layer obtained by vapor deposition of metal such as aluminum on the plastic film, where film exposure portions without any vapor-deposited layer are scattered provided on the surface of the plastic film, the total area of the exposure portions falls within the range of 1/1000 to 1/10 to the surface area of the outer bag (PTL 1). In PTL 1, storage experimentation where three-month storage at a temperature of 40° C. is performed is specifically illustrated. Although this condition indicates that the experimentation illustrated therein is equivalent to 1.5-year storage experimentation at room temperature, it cannot be said to be sufficient in terms of temperature or time. Thus, an actual outer bag for disposable body warmer packaging which can withstand severer storage conditions may be required.
As described above, although various outer bags for disposable body warmer packaging have been proposed, they still leave room for improvement and an outer bag for disposable body warmer packaging and disposable body warmer are required which can ensure an optimum gas permeation property and gas barrier property as a product over a long period.